1. Field of the Invention
The invention relates to a self-compensation device for subtractive detectors.
The aim of the invention is to improve thermal imaging systems using subtractive architecture to eliminate the continuous component of the integrated current. The principle of a subtractive detector is described in the French patent No 2 756 666 and is recalled in FIGS. 1a and 1b. 
2. Description of the Prior Art
As can be seen in a simplified view in FIG. 1a, a detector according to the French patent No 2 756 666 comprises the following elements stacked on a substrate:
a contact layer C2,
an active photoconductive layer D2,
a common contact layer Cc
an active photoconductive layer D1,
and a contact layer C1.
The active photoconductive layers D1, D2 may be layers made of a photoconductive semiconductor material such as silicon. They may also be made in the form of stacks of layers constituting quantum well detectors. The two active layers D1, D2 are photoconductive in the same range of wavelengths xcex. One of the active layers is designed to be highly absorbent in the range of wavelengths xcex while the other layer is designed to absorb very little or be practically non-absorbent. This can be designed by having different thicknesses for the active layers, or by a greater doping of the quantum well layers of the more absorbent active layer. It is possible for the contact layers C1, C2, Cc not to cover the entire surface of the photoconductive layers.
Since the detector is illuminated by the radiation to be detected as can be seen in FIG. 1a, the active layer D2 first receives the radiation RZ.
Should the layer D1 be more absorbent than the layer D2, a diffraction grating is preferably provided. The diffraction grating is associated with the face of the layer D1 bearing the contact layer C1. This grating receives the light that had not been absorbed during the first crossing of the layer D1 and diffracts it towards the layer D1. The diffracted light will be absorbed or almost absorbed by the layer D1.
The contact layers C1 and C2 are used to apply control potentials. The contact layer Cc is common with the two detector elements comprising the active photoconductive layers.
It is set at a reference potential and enables the detection of the photo currents generated by the detector D1, D2.
The substrate is transparent to the range of wavelengths to be measured. The detector therefore receives the radiation RZ through the substrate.
When a radiation RZ is received by the detector, to detect the wavelength (or range of wavelengths) xcex, the following are applied:
a potential V1 to the contact layer C1,
a potential V2 to the contact layer C2:
a floating potential Vc (or ground), between V1 and V2, to the common contact layer Cc.
In the structure D1, the following current flows:
I1=I1d+I1opt
And in the structure D2, the following current flows
I2=I2d+I2opt
The currents I1d and I2d are the dark currents in D1 and D2. The currents I1d and I2d may also represent the sum of a dark current and a current corresponding to the surroundings. The currents I1opt and I2opt are the currents due to the wavelength xcex to be detected in D1 and D2.
In FIG. 1b, the current i collected by the read circuit has the following value:
I=I1xe2x88x92I2
By adjusting the voltage V1 or V2, it is possible to adjust I1d=I2d. The value of the detected current is therefore:
I=I1optxe2x88x92I2opt
By planning the structure so that one of the two active layers absorbs very little energy from the wave xcex, the current I is the one generated by the active layer that has the strongest response.
The total current of a thermal imaging device is the sum of (a) an offset current, constituted by a dark current thermally activated according to a law of the Arrhenius type, I=I0 exp(xe2x88x92hc/xcex)kT), and (b) the current of the optical signal generated by the variations in emissivity and temperature of the scene. The architecture of a subtractive focal plane is used to subtract the continuous component before integration and therefore make full use of the frame time available to integrate the signal without saturating the individual storage capacity of each pixel. This improves the signal-to-noise ratio of the detectors. The two stages QWIP1 and QWIP2 are identical structures. The stage QWIP1, biased at xe2x88x92Vs, is the detection stage and the stage QWIP2, reverse biased at +Vref, is the reference mirror stage, enabling the total or partial subtraction of the current. The intermediate contact is connected to the corresponding storage capacitor of the multiplexer and thus enables the collection of the resulting current, namely the difference in the currents flowing through the two stages.
A thermal imaging device comprises a cooling unit (Stirling machine, Joule-Thomson pressure-reducing device, liquid nitrogen bath etc) and a regulation system capable of stabilizing the temperature of the focal plane T0 to within xc2x1xcex94T. The slow fluctuation of the temperature, which has a variation in amplitude of 2xcex94T, will generate a variation of the thermal current of each of the stages.
The invention can be used to resolve this problem.
The invention therefore relates to a device for the detection of electromagnetic waves comprising at least two photoconductor-based electromagnetic wave detectors, each comprising:
at least two separate, flat-shaped, stacked photoconductor-based active detector elements, comprising a common reading contact, the unit being held between two control contact layers;
means to apply control voltages to each control contact layer, a voltage applied to the common reading contact layer having a value ranging between the voltages applied to the control contact layers;
means connected to the common contact to detect the difference between the photoconduction currents of the detector elements;
wherein at least one detector is provided, on one of its plane faces, with a diffraction grating and wherein a subtraction circuit is used to subtract the read signal of a detector not provided with a diffraction grating from the read signal of a detector provided with a diffraction grating.